Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 10,000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs. Dinosaurs are represented on every continent by both extant species (birds) and fossil remains. Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Most research conducted since the 1970s, however, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction. Some were herbivorous, others carnivorous. Evidence suggests that all dinosaurs were egg-laying; and that nest-building was a trait shared by many dinosaurs, both avian and non-avian.
The Cretaceous–Paleogene extinction event, also known as either the K–Pg extinction event, Cretaceous–Tertiary extinction event, or K–T extinction event, was the large-scale mass extinction of animal and plant species in a geologically short period of time, approximately 66 million years ago (mya). The Cretaceous–Paleogene extinction event is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the K–Pg boundary. K is the traditional abbreviation for the CretaceousPeriod, Pg is the abbreviation for the Paleogene Epoch, and T is the abbreviation for the Tertiary Period. The event marks the end of the Mesozoic Era, and the beginning of the Cenozoic Era.
Non-aviandinosaurfossils are only found below the K–Pg boundary and became extinct immediately before or during the event. A very small number of dinosaur fossils have been found above the K-Pg boundary, but they have been explained as reworked, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through the boundary with few extinctions, and radiation from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms.